Plating method for aluminum alloy material

ABSTRACT

There is provided a method of applying plating to an aluminum alloy material, which comprises a step of cathodic acid electrolysis as a pretreatment for the plating for removing insoluble matters from the aluminum alloy generated in a previous alkaline-etching step.

BACKGROUND OF THE INVENTION

The present invention relates to a method for applying hardwearing plating to an aluminum alloy material. In particular, the present invention relates to a plating method for an aluminum alloy material, which makes it possible to minimize environmental load in the pretreatment for plating.

There are existing methods for applying hardwearing plating, iron plating, to an aluminum alloy material such as a cylinder for an internal combustion engine (the sliding surface of piston) such as a Schnurle scavenging type small air-cooled two-stroke gasoline engine. It has been the conventional practice, as described in JP Patent Laid-open Publication (Kokai) No. 3-191095 (1991), to perform a pretreatment (substrate treatment) in order to enhance the adhesion (biting properties) of plating to the surface of aluminum alloy material prior to the plating process.

This pretreatment has been conducted in a sequence of processes (steps) and under the treatment conditions as shown in the following Table 1. Specifically, the aluminum alloy material is subjected to a degreasing treatment, and the material thus degreased is washed with water and subjected to alkaline etching. The material thus alkali-etched is washed with water and then treated with a mixed solution composed of nitric acid, hydrofluoric acid and sulfuric acid to dissolve and remove (pickling) insoluble matters (smut: Si, Cu, Fe, Mn, etc.) in the material (aluminum alloy) generated (left remaining on the surface layer) in the previous alkaline-etching step. As a result, the surface of the material is made clean, concurrently creating fine recessed and projected portions on the surface of the material. Thereafter, the resultant surface is washed with water and then subjected to an anodic oxidation treatment. As a result, an oxide film is created on the surface of the material and, at the same time, holes are created at portions corresponding to the fine recessed and projected portions of the oxide film. The material having the fine recessed and projected portions formed thereon is washed with water and then the surface of material is subjected to iron plating. In this case, since the iron plating (lowermost portions thereof) is permitted to penetrate into the surface of material so as to bury the holes and the fine recessed and projected portions, it is possible to obtain high anchoring effects, thus rendering the plating excellent in adhesion (biting properties). TABLE 1 (Conventional method) Steps Conditions of treatments Degreasing Bondal Cleaner (Canning Co.): 2.5 g/L, 50° C., 45-55 seconds Water washing Alkali etching Bondal Cleaner (Canning Co.): 25 g/L, 60° C., 45-55 seconds Water washing Pickling Sulfuric acid: 53%; nitric acid: 11%; hydrofluoric acid: 10%; 20-30° C., 45-55 seconds Water washing Anodic oxidation Phosphoric acid: 50 g/L, 55° C., 50 seconds; Voltage: 50 V Water washing Plating Iron plating

However, the aforementioned conventional plating method has the following problems.

According to the conventional plating method, insoluble matters (smut) in an aluminum alloy generated in the alkaline-etching step are dissolved and removed (pickling) by making use of a mixed solution composed of nitric acid, hydrofluoric acid and sulfuric acid. However, the use of nitric acid in this pickling step is undesirable because of the regulations on nitrogen in view of the problem of eutrophication of sea water. The use of hydrofluoric acid is also undesirable since hydrofluoric acid is a Class 1 designated substance specified by the PRTR (Pollutant Release and Transfer Register). Therefore, there is a strong demand to restrict the use of nitric acid as well as hydrofluoric acid in order to minimize the negative environmental effects. Accordingly, it is strongly desired to develop a method of applying plating (pretreatment) to aluminum alloy materials which does not necessitate the use of nitric acid and hydrofluoric acid.

Furthermore, the nonuse of nitric acid and hydrofluoric acid as much as possible is desirable not only for the purpose of avoiding the natural environmental problems mentioned above, but also for the well-being of workers, e.g., for ensuring the safety of workers dealing with the pretreatment for plating.

The conventional plating method is also problematic because the desmutting treatment by means of pickling mentioned above is low in stability and hence the depth of etching (which has much to do with the adhesion of plating) tends to fluctuate. The administration of the desmutting treatment is very difficult because of its instability. As a result, it is often impossible to secure sufficient anchoring effects on the plating that has been applied to the aluminum alloy material, thus rendering the plating defective as regards adhesion.

Additionally, because the chemicals (a mixed solution composed of nitric acid, hydrofluoric acid and sulfuric acid) employed in the pickling have a short life-span and because transforming the waste water containing hydrofluoric acid into harmless water takes a considerably long time, the cost of the chemicals and that for the waste water treatment are high.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in view of the problems mentioned above and, therefore, some of the objects of the present invention are to provide a method of applying plating to an aluminum alloy material which makes it unnecessary to employ nitric acid and hydrofluoric acid in the pretreatment for plating, which in turn minimizes the negative environmental effects, improves the working environment for pretreatment personnel, reduces the costs for chemicals and wastewater treatment, and secures excellent adhesion of the plated layer.

With a view to achieving the aforementioned objects, there is provided, according to one aspect of the present invention, a method of applying plating to an aluminum alloy material, which essentially comprises a step of cathodic acid electrolysis as a pretreatment for the plating for removing insoluble matters in the aluminum alloy generated in a previous alkaline-etching step.

In one embodiment, the method of applying plating to an aluminum alloy material comprises the steps of: degreasing the aluminum alloy material; subjecting the aluminum alloy material thus degreased to an alkali-etching treatment; subjecting the aluminum alloy material thus alkali-etched to cathodic acid electrolysis; subjecting the aluminum alloy material thus electrolyzed to an anodic oxidation treatment; and applying plating to the aluminum alloy material thus anodically oxidized.

Preferably, the cathodic acid electrolysis is performed in a solution of ferric sulfate with the aluminum alloy material being employed as a cathode.

The aluminum alloy material may be a cylinder for an internal combustion engine (e.g., a cylinder for a small air-cooled two-stroke gasoline engine) wherein the plating is applied to the sliding surface of the piston of the cylinder.

The cylinder for an internal combustion engine according to another aspect of the present invention is featured in that an iron plating is applied to the sliding surface of the piston according to the aforementioned plating method.

The method of applying plating to an aluminum alloy material according to the present invention is featured in that the insoluble matters (smut: Si, Cu, Fe, Mn, etc.) generated in the previous alkaline-etching step are removed not by means of pickling (which has been conventionally employed as a pretreatment for plating) but by means of cathodic acid electrolysis. More specifically, the aluminum alloy material is employed as an anode, and the cathodic acid electrolysis is performed in a solution of ferric sulfate to thereby physically remove the insoluble matters by taking advantage of hydrogen gas to be generated in the acid electrolysis. Therefore, it is no longer necessary to employ nitric acid and hydrofluoric acid in the pretreatment for plating. As a result, it is now possible to minimize the environmental load, to improve the working environment for pretreatment personnel, and to reduce the costs for chemicals and waste water treatment.

Furthermore, since the depth of etching is determined by the treatment conditions (the concentration of a chemical to be employed (caustic soda, etc.), temperature and time) in the alkaline-etching step, and the etching does not proceed in the step of cathodic acid electrolysis as only hydrogen gas is permitted to generate in the cathodic acid electrolysis, it is possible to easily and accurately control the depth of etching. As a result, it is possible to realize high anchoring effects of the plated layer that has been applied to the aluminum alloy material, resulting in improved adhesion (biting property) of plating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron microphotograph illustrating the surface of aluminum alloy material after a degreasing treatment in one embodiment of the method of applying plating to an aluminum alloy material according to the present invention.

FIG. 2 is a scanning electron microphotograph illustrating the surface of aluminum alloy material after an alkaline-etching treatment in one embodiment of the method of applying plating to an aluminum alloy material according to the present invention.

FIG. 3 is a scanning electron microphotograph illustrating the surface of aluminum alloy material after a cathodic acid electrolysis in one embodiment of the method of applying plating to an aluminum alloy material according to the present invention.

FIG. 4 is a scanning electron microphotograph of a cross-section of the surface of aluminum alloy material after an Fe plating in one embodiment of the method of applying plating to an aluminum alloy material according to the present invention.

FIG. 5 is a cross-sectional view of a Schnurle scavenging type small air-cooled two-stroke gasoline engine.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment representing the plating method according to the present invention is directed to the case where hardwearing plating (iron plating) 4 is to be applied to an aluminum alloy material, for example, to a sliding surface 3 of the piston 2 of a cylinder (die cast cylinder of ADC12) 1 as shown in FIG. 5 for a Schnurle scavenging type small air-cooled two-stroke gasoline engine which is designed to be employed in a portable power tool such as a brush cutter. This plating method is performed in a sequence of processes (steps) and under the treatment conditions as shown in the following Table 2. TABLE 2 Steps Conditions of treatments Degreasing Bondal Cleaner (Canning Co.): 2.5 g/L, 50° C., 45-55 seconds Water washing Alkali etching Bondal Cleaner (Canning Co.): 25 g/L, 60° C., 45-55 seconds Water washing Cathodic Fe agent (Chuou Kagaku Co.): 30%; EBR agent: 3%; electrolysis 25-60° C., 30-90 seconds; Voltage: 6-12 V Water washing Anodic oxidation Phosphoric acid: 50 g/L, 55° C., 50 seconds; Voltage: 50 V Water washing Plating Iron plating

First, the aluminum alloy material (the sliding surface 4 of piston 2 of the cylinder 1) is subjected to a degreasing treatment. This degreasing treatment is performed, as in the case of the conventional plating method mentioned above (see Table 1), for 45-55 seconds at a temperature of 50° C. by making use of Bondal Cleaner (caustic soda: 2.5 g/L; Canning Co.) for example. In this degreasing treatment, only degreasing takes place as shown in the photograph of FIG. 1 (a scanning electron microphotograph illustrating the surface of aluminum alloy material after the degreasing treatment: SEM×1600). There is no etching of the aluminum alloy material during this step.

The material 1 thus degreased is washed with water and then subjected to alkaline etching. This alkaline etching is also performed, as in the case of the conventional plating method mentioned above (see Table 1), for approximately 45-55 seconds at a temperature of 60° C. by making use of Bondal Cleaner (caustic soda: 25 g/L; Canning Co.) for example.

In this alkaline-etching step, the etching of the surface of the material 1 is allowed to proceed by the effect of caustic soda as shown in the photograph of FIG. 2 (a scanning electron microphotograph illustrating the surface of aluminum alloy material after the alkaline etching: SEM×1600). As a result, insoluble matters (Si, Cu, Fe, Mn, etc.) in the aluminum alloy are left as a smut on the surface of the material 1.

Thereafter, the material 1 thus alkali-etched is washed with water and then subjected to cathodic acid electrolysis in order to remove insoluble matters (smut: Si, Cu, Fe, Mn, etc.) in the material (aluminum alloy) 1 generated (left remaining on the surface layer) in the alkaline-etching step. More specifically, this cathodic acid electrolysis is performed for 30 to 90 seconds in an aqueous solution (a ferric sulfate solution) comprising 30% of Fe agent (descaling agent comprising ferric sulfate, nonionic surfactant and sulfuric acid; Chuou Kagaku Co., Ltd.), and 3% of EBR agent (an additive for acid electrolysis) under the conditions of: 25-60° C. in temperature and 6-12V in voltage with the material 1 being employed as a cathode. In this cathodic acid electrolysis, the smut is removed by the effect of hydrogen gas generated from the cathode as shown in the photograph of FIG. 3 (a scanning electron microphotograph illustrating the surface of aluminum alloy material after the cathodic acid electrolysis: SEM×1600). As a result, the surface of the material 1 is made clean, concurrently creating fine recessed and projected portions which are best suited for the improvement of adhesion of plating (enhanced anchoring effects) on the surface of the material 1.

Thereafter, the resultant surface is washed with water and then subjected to an anodic oxidation treatment. This anodic oxidation treatment is performed in the same manner as the conventional plating method mentioned above (see Table 1). Specifically, this anodic oxidation treatment is performed for approximately 50 seconds using phosphoric acid 50 g/L under the conditions of: 55° C. in temperature and 6-12V in voltage. As a result, an oxide film is created on the surface 3 of the material and, at the same time, holes are created at portions corresponding to the fine recessed and projected portions of the oxide film.

The material 1 having the fine recessed and projected portions formed thereon is washed with water and then the surface of material 1 is subjected to iron plating 4. In this case, the iron plating 4 (lowermost portions thereof) is permitted to penetrate into the surface of material 1 so as to bury the holes and the fine recessed and projected portions as shown in the photograph of FIG. 4 (a scanning electron microphotograph illustrating the surface of aluminum alloy material after the cathodic acid electrolysis: SEM×1150). As a result, it is possible to obtain high anchoring effects of the plating 4, thus rendering the plating 4 excellent in adhesion (biting properties).

As described above, in the plating method of this embodiment, the insoluble matters (smut: Si, Cu, Fe, Mn, etc.) generated in the previous alkaline-etching step are removed by making use of cathodic acid electrolysis substituting for the pickling which has been conducted in the conventional pretreatment for plating. More specifically, the aluminum alloy material 1 is employed as an anode, and the cathodic acid electrolysis is performed in a solution of ferric sulfate to thereby physically remove the insoluble matters by taking advantage of hydrogen gas generated in the acid electrolysis. Therefore, it is no longer necessary to employ nitric acid and hydrofluoric acid in the pretreatment for plating. As a result, it is now possible to minimize the environmental load, to improve the working environment for pretreatment personnel, and to reduce the costs for chemicals and wastewater treatment.

Furthermore, since the depth of etching is determined by the treatment conditions (the concentration of a chemical to be employed (caustic soda, etc.), temperature and time) in the alkaline-etching step, and the etching does not proceed in the step of cathodic acid electrolysis as only hydrogen gas is permitted to generate in the cathodic acid electrolysis, it is possible to easily and accurately control the depth of etching. As a result, it is possible to realize high anchoring effects of the plated layer that has been applied to the aluminum alloy material, resulting in improved adhesion (biting property) of plating.

In the foregoing embodiments, although the present invention has been explained in detail with reference to one embodiment, the present invention should not be construed as being limited to this embodiment. For example, the kinds and quantities of chemicals to be employed in each of the steps, as well as other treatment conditions, can be variously modified depending on the specifications, etc., of plating as required. 

1. A method of applying plating to an aluminum alloy material, which comprises a step of cathodic acid electrolysis as a pretreatment for the plating for removing insoluble matters in the aluminum alloy generated in a previous alkaline-etching step.
 2. The method according to claim 1, wherein the cathodic acid electrolysis is performed in a solution of ferric sulfate with the aluminum alloy material being employed as a cathode.
 3. The method according to claim 1, wherein the aluminum alloy material is a cylinder for an internal combustion engine wherein the plating is applied to a sliding surface of piston of the cylinder.
 4. The method of claim 3 wherein the iron plating is applied to the sliding surface of the piston.
 5. A cylinder for an internal combustion engine, which is featured in that a sliding surface of piston thereof is applied with an iron plating according to the method claimed in claim
 3. 6. A method of applying plating to an aluminum alloy material, which comprises the steps of: degreasing the aluminum alloy material; subjecting the degreased aluminum alloy material to an alkali-etching treatment; subjecting the alkali-etched aluminum alloy material to cathodic acid electrolysis; subjecting the electrolyzed aluminum alloy material to an anodic oxidation treatment; and applying plating to the anodically oxidized aluminum alloy material.
 7. The method according to claim 6, wherein the cathodic acid electrolysis is performed in a solution of ferric sulfate with the aluminum alloy material being employed as a cathode.
 8. The method according to claim 6, wherein the aluminum alloy material is a cylinder for an internal combustion engine wherein the plating is applied to a sliding surface of a piston of the cylinder.
 9. The method of claim 8 wherein the iron plating is applied to the sliding surface of the piston.
 10. A cylinder for an internal combustion engine, which is featured in that a sliding surface of piston thereof is applied with an iron plating according to the method claimed in claim
 8. 